Reduced isotropic etchant material consumption and waste generation

ABSTRACT

Methods and apparatus for isotropically etching a metal from a work piece, while recovering and reconstituting the chemical etchant are described. Various embodiments include apparatus and methods for etching where the recovered and reconstituted etchant is reused in a continuous loop recirculation scheme. Steady state conditions can be achieved where these processes are repeated over and over with occasional bleed and feed to replenish reagents and/or adjust parameters such as pH, ionic strength, salinity and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalApplication Ser. No. 61/239,350 filed Sep. 2, 2009, the contents ofwhich are incorporated herein by reference in their entirety and for allpurposes.

FIELD OF THE INVENTION

This invention pertains to methods of wet chemical etching. Moreparticularly this invention pertains to methods and apparatus forregenerating and reusing wet etchant for copper removal andplanarization in semiconductor processing.

BACKGROUND

Isotropic etching is non-directional removal of material from asubstrate via a chemical process using an etchant. Etchants can includeliquids and plasmas. Liquid chemical etchants are typically corrosive,containing acids or bases and other agents to enhance the etchantsability to remove material from a work piece. Such etchants are used,for example, to efficiently remove unwanted material from a work piece.Isotropic etching is particularly useful for removing unwanted metal,for example copper, from semiconductor wafers.

Isotropic etching metal from a work piece typically produces largevolumes of waste, on the order of tens of liters per hour, for example,to process semiconductor wafers through a single etch apparatus. Thiswaste, although moderately dilute, can contain many environmentalpoisons including metal ions, for example copper. Also, feed stocks forcreating the liquid etchant are expensive. Handling large volumes ofcaustic and toxic waste presents a major challenge, for example, insemiconductor processing where large numbers of work pieces areprocessed daily.

SUMMARY

Methods and apparatus for isotropically etching a metal from a workpiece, while recovering and reconstituting the chemical etchant aredescribed. Various embodiments include apparatus and methods for etchingwhere the recovered and reconstituted etchant is reused in a continuousloop recirculation scheme. Steady state conditions can be achieved wherethese processes are repeated over and over with occasional bleed andfeed to replenish reagents and/or adjust parameters such as pH, ionicstrength, salinity and the like. This saves from having to process hugewaste streams and takes advantage of synergies between processes asdescribed herein.

One embodiment is an apparatus for processing a work piece, including:(a) a wet chemical etching chamber for removing a metal from a surfaceof the work piece with a peroxide-based etchant, the wet chemicaletching chamber in fluid communication with and upstream of; (b) anelectrowinning module for removing ions of the metal from theperoxide-based etchant after it exits the wet chemical etching chamber;and (c) a regeneration system configured to reintroduce one or morereagents into the peroxide-based etchant after it exits the wet chemicaletching chamber in order to regenerate the peroxide-based etchant andreintroduce it into the wet chemical etching chamber. Apparatus mayfurther include a decomposition tank configured to allow substantialdecomposition of a peroxide in the peroxide-based etchant after exitingthe wet chemical etching chamber and before entering the electrowinningmodule. Regeneration systems may include: a peroxide inlet forintroducing the peroxide into the peroxide-based etchant after theperoxide-based etchant exits the electrowinning module, the peroxideinlet proximate to an etchant inlet of the wet chemical etching chamber;and one or more make-up feeds for adding at least one of water, a metalchelator and a pH adjuster to the peroxide-based etchant. Apparatus mayfurther include a buffer tank configured to store a volume of theperoxide-based etchant after it exits the electrowinning module, thebuffer tank configured upstream of the peroxide inlet.

Another embodiment is a method for processing a work piece, including:(a) wet chemical etching a metal from a surface of the work piece with aperoxide-based etchant; (b) electrowinning ions of the metal from theperoxide-based etchant after wet chemical etching; (c) regenerating theperoxide-based etchant by adding one or more reagents to theelectrowinned peroxide-based etchant; and (d) reusing the regeneratedperoxide-based etchant for wet chemical etching. Methods may furtherinclude passing the peroxide-based etchant through a decomposition tank,after wet chemical etching and before electrowinning, in order todecompose a peroxide in the peroxide-based etchant. Methods describedherein are particularly useful for etching copper. Copper concentrationsfrom the used etchant solution are reduced to low levels, in oneembodiment, less than about 200 ppm of copper ions, in anotherembodiment less than about 100 ppm of copper ions. One or more reagentsadded to the stabilized and electrowinned etchant include least one ofwater, a peroxide, a metal chelator and a pH adjuster. In oneembodiment, the pH adjuster includes an organic and/or and inorganicbase. Methods of monitoring the etch, decomposition and/orelectrowinning are described where data collected is used to maintain asubstantially steady state etchant composition for etching in arecirculation format where the used etchant is regenerated and usedagain. In one embodiment, determining how much of each, of the one ormore reagents to add to the electrowinned peroxide based etchant isachieved by analyzing the electrowinned peroxide-based etchant todetermine at least one of pH, salt concentration, peroxideconcentration, metal ion concentration, ionic strength, chelatorconcentration and the like.

Another embodiment is a method of reducing the volume of a waste streamproduced by etching copper with a chemical etchant, including: (a)electrowinning copper ions from the waste stream until a copper ionconcentration of about 200 ppm or less is reached; and (b)reconstituting the chemical etchant using the electrowinned waste streamand one or more reagents; and (c) reusing the reconstituted chemicaletchant to further etch copper.

These and other features and advantages are further discussed below withreference to the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the solution temperature vs. time of an etchsolution after the addition of hydrogen peroxide.

FIG. 2 is a plot of estimated time of the etch solution described inrelation to FIG. 1 to decompose as a function of pH.

FIG. 3 is a pH titration curve of the etch solution from FIG. 1 usingcopper plating solution as the titrant.

FIG. 4 shows the general layout of an electrowinning apparatus.

FIG. 5 is a process flow depicting aspects of a method of processing awork piece.

FIG. 6 is a schematic of an apparatus for processing a work piece.

FIG. 7 shows the results of copper concentration in the etch solutionvs. time using an experimental electrowinning apparatus.

DETAILED DESCRIPTION

While methods for efficient copper removal are desirable at variousstages of semiconductor device fabrication, conventional wet copperetching techniques have not been widely introduced because of theiranisotropic nature. Anisotropic etching leads to preferential etching ofcopper in one specific direction and/or preferential etching of one typeof grain orientation and, consequently, leads to roughening of coppersurface, pitting, and grain boundary dependent non-uniform copperremoval. Therefore isotropic removal of copper is commonly desired.

Exemplary isotropic etchants, methods and apparatus are described in thefollowing: U.S. Pat. No. 7,531,463, filed Oct. 24, 2006 (Koos, et. al.),U.S. Pat. No. 7,338,908, filed Oct. 20, 2003 (Koos, et. al.), U.S.patent application Ser. No. 11/602,128, filed Nov. 20, 2006 (Mayer etal.), U.S. patent application Ser. No. 11/888,312, filed Jul. 30, 2007(Mayer et al.), U.S. patent application Ser. No. 12/462,424, filed Aug.4, 2009 (Mayer et al.), and U.S. patent application Ser. No. 12/535,594,filed Aug. 4, 2009 (Mayer et al.), each of which is herein incorporatedby reference in its entirety.

Isotropic etch systems typically create an in-line a mixture of “fresh”metal etching chemicals that are used once (single pass) and then thespent solution is treated as waste. For example, an etch stream iscreated by combining a solvent, for example water, an isotropic copperchelator concentrate, typically containing one or more amino acidsand/or di-, tri- and tetra-amines (e.g. glycine, ethylene diamine), andtypically one or more neutralizing agents. A specific example of acopper chelator concentrate is Novellus composition ChemX-GSA2™,available though ATMI Industries of Danbury, Conn. The concentrate andwater streams are mixed with an oxidizer, typically a peroxide, forexample 30% hydrogen peroxide, to form an active isotropic etchsolution.

The isotropic etch may have a number of very favorable characteristics,including grain independent and feature size, depth and densityindependency, as well as minimal surface roughening (i.e. the surfaceremains bright, even after removing 10 or more microns of metal). Thecosts of the raw chemicals and the volume of total waste generated arevery favorable versus the major competitive processes, for examplecopper chemical mechanical polishing (CMP).

Among the current technical issues with isotropic etching is the need toprocess and eventually treat a relatively large liquid waste volume thatis moderately dilute. Also, the waste stream contains an environmentalpoison, copper. In some cases, this stream may be as large as 25 litersper hour from a single processing module (e.g. if the etch process mustremove 40-50 μm of plated metal), and has a copper content in the rangeof about 0.1%, or 1000 ppm. For comparative purposes, this waste streamvolume is about 10-50 times that of a typical Sabre™ plating tool (acopper electrofill tool available from Novellus Systems, Inc. of SanJose, Calif.), which produces on the order of 0.62-1.25 liters per hourof concentrated waste (about 4% copper, or 40,000 ppm) and is about ⅓the volume of the Sabre EBR™ (edge bevel removal tool, also availablefrom Novellus Systems, Inc. of San Jose, Calif.) which produces about100 liters per hour of rinse waste (10-50 ppm copper). The exact valuesof course depend on tool particular application, throughput,utilization, and other operating conditions.

In a typical waste treatment scheme not involving the etch solution,concentrated plating solution is typically first kept as a separatewaste stream from the rinse waste, and may be electrowinned down to acopper concentration of less than about 200 ppm. Then this small volumeacidic waste is mixed with the larger volume, less acidic, low copperrinsate stream, the combined streams are pH modified to between about pH4 and about 5, where they are then passed though a series ion exchangeresins, exiting the treatment as a near neutral low copper content(typically between 1-5 ppm) waste stream.

The mixing of the plating waste streams with etch waste streams, havinga very different volumes, metal concentrations, pH, oxidizer content andother constituents, was found to be not fully compatible, oreconomically and environmentally optimal.

The inventors have found that, in addition to being non-optimal tosimply mix the plating tool waste with the etch waste because of theirincongruent volume and metal ion concentration, the etch solution isalkaline (pH 8-11) and contains an oxidizer and hence has otherincompatibilities. This difference in the waste streams presents someunique problems. It was recognized as favorable (from a waste treatmentperspective) that the hydrogen peroxide oxidizer in the etch solutionbreaks down rapidly (in about 10 minutes) to oxygen and water andtherefore is a stable waste product. The decomposition process isexothermic, creating oxygen and water, and can be monitored by measuringthe adiabatic temperature rise of a decomposing solution, the results ofsuch measurement are presented in equation (1).

H₂O₂H₂O→½O₂ ΔH_(react)=−23.4 kcal/mole  (1)

The decomposition of a typical etch solution, for example a 4 percentsolution by weight (1.18 M) of hydrogen peroxide solution, would release23.4×1.18=−27.6 kcal/liter and have a heat capacity of about 0.98Kcal/L, so the adiabatic temperature increase for the completedecomposition of such a solution would be as in equation (2), a value

ΔT=−ΔH/C_(P)=27.6/0.98=28.2° C.  (2)

commensurate with measurements where the decomposition is rapid (nearlyadiabatic). However, the oxidizer break down rate is a strong non-linearfunction of pH (described in more detail below), and therefore simplycombining the etch waste with various acidic waste streams can create anumber of challenges in chemical handling, treatment and safety.

By measuring the time to reach a maximum temperature one can approximatethis value with the average time (rate) of the peroxide decomposition.FIG. 1 is a plot of the solution temperature vs. time of an etchsolution after the addition of hydrogen peroxide. The solutions weremade by combining a) 80 mL of an etch concentrate electrolytic“precursor”, b) 0 or 1.3 g metallic copper powder, c) 800 ml ofdeionized (DI) water and, just prior to starting the experiment, d) 133g (120 ml) of 30% hydrogen peroxide in water. One liter of the etchconcentrate contained approximately 231 ml/L 98% ethylene diamine, 198g/L glycine, 8.8 ml/L glacial acetic acid, 3.9 ml/L 96% sulfuric acidand 718 mL of DI water. This combination of a-d initially creates a etchsolution having 4%/wt hydrogen peroxide, 8% etch concentrate, and, afterreaction, contains either 0 or 1300 ppm dissolved copper. The lattercopper concentration represents a typical copper concentration of theused etch solution after etching copper from a 300 mm wafer. Thus FIG. 1shows comparison of decomposition of 4% H₂O₂, 8% etch concentratesolution in water, approximating the composition of isotropic etchwaste, with and without copper. Referring to FIG. 1, the maximumtemperature is reached in about 7 minutes without copper in thesolution, and in about 4.5 minutes with 1300 ppm of copper, indicatingthat copper may catalyze the rate of hydrogen peroxide decomposition.Bubbling of the solution is clearly visible during this period prior toreaching the temperature peak.

FIG. 2 is a plot of the decomposition of the etch solution described inrelation to FIG. 1 as a function of pH. The pH was adjusted by titratingthe etch solution before adding the peroxide with a copper platingsolution (pH of about 0.45, 20 g/L methane sulphonic acid, 80 g/L coppermethane sulphonate, 50 ppm Cl⁻). FIG. 3 is the pH titration curve of theetch solution using the copper plating solution as the titrating agent.The estimate of the peroxide decomposition over time is made frommeasuring the time to maximum temperature (similar to FIG. 1), or, inthe case of very long decomposition times, the measured time for thesolution etch rate of copper to drop below 50 Å/min. At the lower pHranges the acidity tends to reduce the rate of decomposition, but theincreasing copper is likely compensating for this reduction bycatalyzing the decomposition.

What can be concluded from FIGS. 2 and 3 is that at some conditions(near pH 7 and at low pH with high copper), the peroxide breakdown rateis relatively slow. This creates a potential for waste which exits thetool to have its rate of decomposition reduced and be temporarily“stabilized” by mixing with acidic plating or rinse waste. However, ifthe pH of the mixed waste were to later change (e.g. from pH 7 to pH 4,or from pH<3 to pH 4-6) the stored peroxide in the solution and heat andgas would be rapidly released in a waste storage tank. Avoiding such anunpredictable and potentially hazardous situation is thereforedesirable. Also, the inventors have found that once the peroxide in theused etch solution is decomposed, the remaining components of the etchsolution are essentially intact, except that the solution containshigher than desired copper concentration. Thus surprisingly, theinventors have found that if such etch solutions are “stabilized” bydecomposition of the peroxide, and the copper removed, for example byelectrowinning down to an acceptable level, for example less than about200 ppm, then the resulting solution can be used to regenerate the etchsolution by adding fresh hydrogen peroxide. In certain embodiments, thiscycle is repeated in a loop fashion, with some maintenance of therecycling loop, for example, occasionally bleeding off some of thesolution and adding pH adjusters, water, amines and the like, tomaintain a substantially steady state etchant composition in a recyclingloop format.

In one embodiment, a general etch solution recycling method along withsome exemplary process schemes and associated apparatus, are useful inreducing the amount of chemical used and waste created in an isotropicmetal removal etch process by about 90% or more are described. Incertain embodiments, the method and apparatus can be performed on- oroff-board a semiconductor plating tool. In one example, a recyclingmethod and associated exemplary apparatus includes four core operationsand/or components: 1) a metal (e.g. copper) etching chamber (e.g. Sabreetching module) which uses a stream (e.g. spray) of (preferablyisotropic) etchant containing a solvent (e.g. water), which is a carrierof the etch chelating chemical(s) and pH adjusting component(s), anddissolved oxidizer(s), 2) An etch oxidizer decomposition tank suitablydesigned to allow for most or substantially all of the peroxidecomponent of the etch solution to be broken down and the break-downproduct separated from the etch solution, 3) an electrowinning cell (orother suitable cell that removes dissolved metal etched from the wafersurface in the etching chamber) and deposits the metal on, e.g., aseries of porous flow though cathode collector plates, theelectrowinning creating a low metal content solution that replenishesetch chelating solution, and 4) a recirculation system includingmechanisms for pumping the replenished etch chelating solution obtainedafter processes (2) and (3) and combining the reclaimed chemical in-linewith fresh oxidizing agent (e.g. Hydrogen peroxide), so that it can bereused to etch subsequent wafers. Various process monitoring, modifyingand controlling, and a bleed and feed capability are also described.

Thus, one embodiment is a method for processing a work piece, including:(a) wet chemical etching a metal from a surface of the work piece with aperoxide-based etchant; (b) removing, for example by electrowinning,ions of the metal from the peroxide-based etchant after wet chemicaletching; (c) regenerating the peroxide-based etchant by adding one ormore reagents to the electrowinned peroxide-based etchant; and (d)reusing the regenerated peroxide-based etchant for wet chemical etching.In one embodiment, the method further includes passing theperoxide-based etchant through a decomposition tank, after wet chemicaletching and before removing the metal ions, for example byelectrowinning, in order to decompose a peroxide in the peroxide-basedetchant. In one embodiment, electrowinning is used to remove the metalions. In one embodiment, the metal is copper. In another embodiment,greater than about 50% of the peroxide in the peroxide-based etchant isdecomposed without applying heat or additional reagents to theperoxide-based etchant in the decomposition tank. In another embodiment,greater than about 50% of the peroxide in the peroxide-based etchant isdecomposed and a decomposition catalyst is added to the peroxide-basedetchant. In another embodiment, where electrowinning is used, theelectrowinning is performed until the peroxide-based etchant includesless than about 200 ppm of copper ions, in another embodiment less thanabout 100 ppm of copper ions. In one embodiment, the one or morereagents includes at least one of water, a peroxide, a metal chelatorand a pH adjuster. In one embodiment, the peroxide is hydrogen peroxide.In another embodiment, the metal chelator is an amine. In thisdescription, the term “amine” refers to a molecule containing an aminofunctionality, for example, ethylenediamine, ammonia, aniline, and aminoacids are all amines. In one embodiment, the amines (and etch solutions)are described in the aforementioned one or more patents and patentapplications incorporated by reference above.

In one embodiment, the method further includes removing a portion of theperoxide-based etchant to compensate for the addition of the one or morereagents to the electrowinned peroxide-based etchant. In anotherembodiment, the method further includes determining which, and how muchof each, of the one or more reagents to add to the electrowinnedperoxide based etchant by analyzing the electrowinned peroxide-basedetchant to determine at least one of pH, salt concentration, peroxideconcentration, metal ion concentration, ionic strength and chelatorconcentration; and adding the one or more reagents to the electrowinnedperoxide based etchant and/or removing the portion of the peroxide-basedetchant in order to maintain a steady state composition of theelectrowinned peroxide based etchant. In one embodiment, (a)-(d) aboveare performed in a recirculating fashion.

One embodiment is a method for processing a semiconductor wafer,including: (a) wet chemical etching copper from the semiconductor waferwith an etchant including hydrogen peroxide; (b) stabilizing the etchantafter wet chemical etching by decomposing hydrogen peroxide in theetchant; (c) electrowinning copper ions from the etchant afterstabilization; (d) regenerating the etchant by adding one or morereagents to the stabilized and electrowinned etchant; and (e) reusingthe regenerated etchant for further wet chemical etching of copper.

Another embodiment is a method of reducing the volume of a waste streamproduced by etching copper with a chemical etchant, including: (a)electrowinning copper ions from the waste stream until a copper ionconcentration of about 200 ppm or less is reached; and (b)reconstituting the chemical etchant using the electrowinned waste streamand one or more reagents; and (c) reusing the reconstituted chemicaletchant to further etch copper. In one embodiment, (a)-(c) is carriedout on a waste stream created from further etching copper with thereconstituted chemical etchant. In one embodiment, the chemical etchantincludes hydrogen peroxide and the hydrogen peroxide remaining in thewaste stream is first decomposed before (a). In one embodiment, the oneor more reagents includes at least one of water, a peroxide, a metalchelator and a pH adjuster.

Apparatus are also described. One embodiment is an apparatus forprocessing a work piece, including: (a) a wet chemical etching chamberfor removing a metal from a surface of the work piece with aperoxide-based etchant, the wet chemical etching chamber in fluidcommunication with and upstream of; (b) an electrowinning module forremoving ions of the metal from the peroxide-based etchant after itexits the wet chemical etching chamber; and (c) a regeneration systemconfigured to reintroduce one or more reagents into the peroxide-basedetchant after it exits the wet chemical etching chamber in order toregenerate the peroxide-based etchant and reintroduce it into the wetchemical etching chamber. In one embodiment, the apparatus furtherincludes a decomposition tank configured to allow substantialdecomposition of a peroxide in the peroxide-based etchant after exitingthe wet chemical etching chamber and before entering the electrowinningmodule. In one embodiment, the regeneration system includes: a peroxideinlet for introducing the peroxide into the peroxide-based etchant afterthe peroxide-based etchant exits the electrowinning module, the peroxideinlet proximate to an etchant inlet of the wet chemical etching chamber;and one or more make-up feeds for adding at least one of water, a metalchelator and a pH adjuster to the peroxide-based etchant. In oneembodiment, the apparatus further includes a buffer tank configured tostore a volume of the peroxide-based etchant after it exits theelectrowinning module, the buffer tank configured upstream of theperoxide inlet. In one embodiment, the decomposition tank is configuredto hold the peroxide-based etchant for sufficient time such that greaterthan about 50% of the peroxide in the peroxide-based etchant decomposeswithout applying heat or additional reagents to the peroxide-basedetchant in the decomposition tank. In one embodiment, the peroxide-basedetchant includes an amine metal chelator and the peroxide is hydrogenperoxide.

In another embodiment, the metal is copper and the work piece is asemiconductor wafer. In one embodiment, the electrowinning module isconfigured to remove copper ions from the peroxide-based etchant so thatthe peroxide-based etchant exiting the electrowinning module includesless than about 200 ppm of copper ions, in another embodiment, less thanabout 100 ppm of copper ions.

In one embodiment, the apparatus is configured to deliver regeneratedperoxide-based etchant to the wet chemical etching chamber in less thanabout 3 minutes after mixing. In another embodiment, the apparatusincludes a bleed valve for removing the peroxide-based etchant from theapparatus to compensate for the one or more make-up feeds adding the atleast one of water, a metal chelator and a pH adjuster to theperoxide-based etchant.

Apparatus can also include a buffer tank configured to store a volume ofthe peroxide-based etchant after it exits the decomposition tank, thebuffer tank configured upstream of the electrowinning module. Additionalfeatures include one or more analytical probes configured to measure theperoxide-based etchant in the electrowinning module for at least one ofpH, salt concentration, peroxide concentration, metal ion concentration,ionic strength and chelator concentration. One or more controllers canbe included, each with an associated logic, the controller configured tooperate at least the bleed valve and the one or more make-up feeds inorder to maintain a steady state in the composition of theperoxide-based etchant as it exits the electrowinning module usingmeasurements obtained from the one or more analytical probes.

Another embodiment is an apparatus for processing a semiconductor wafer,including: (a) a wet chemical etching chamber for removing copper fromthe semiconductor wafer with an etchant including hydrogen peroxide, thewet chemical etching chamber in fluid communication with and upstreamof; (b) a decomposition tank configured to allow substantialdecomposition of hydrogen peroxide in the etchant after the etchantexits the wet chemical etching chamber and before the etchant enters;(c) an electrowinning cell for removing copper ions from the etchant;(d) one or more make-up feeds for adding at least one of water, a metalchelator and a pH adjuster to the etchant in, and/or downstream of, theelectrowinning cell; and (e) a peroxide inlet for introducing hydrogenperoxide into the etchant after the etchant exits the electrowinningmodule, the peroxide inlet proximate to an etchant inlet of the wetchemical etching chamber. In one embodiment, the apparatus furtherincludes buffer tank configured to store a volume of the etchant afterit exits the electrowinning cell, the buffer tank configured upstream ofthe peroxide inlet. FIG. 6, depicts such an apparatus, 600, forprocessing a work piece in accord with the method described in FIG. 5.FIG. 5 depicts a process flow, 500, describing aspects of a method ofprocessing a work piece. FIG. 5 is described along with FIG. 6. Themethods and apparatus described with respect to FIGS. 5 and 6 are onlyillustrative, the invention is not so limited. For example, a peroxidebased etchant can use hydrogen peroxide and/or other peroxides.

Process flow 500 begins with wet chemical etching a work piece to removea metal, see 505. As discussed, isotropic etching solutions,particularly peroxide based alkaline etchants, are well suited for thismethod, particularly when the metal is copper. Referring to FIG. 6,apparatus 600 includes a wet etch module, 605, in which the chemicaletchant is applied, for example sprayed, onto a wafer to isotropicallyremove a portion of copper on the surface of a wafer. Exemplary etchmodules are described in the U.S. patents and applications incorporatedby reference above.

As described, some embodiments include peroxide decomposition. Referringto FIG. 6, used etchant flows from etch module 605, for example viagravity feed, to a peroxide decomposition tank, 610. The peroxide in aperoxide based alkaline etchant will eventually decompose naturally.However, it is desirable to incorporate a buffer tank in which theperoxide decomposition is facilitated. In one such embodiment, to avoidthe above identified peroxide stability and treatment problem, a wasteetch solution decomposition “buffer holding” tank is provided in theline, for example, integrated into the apparatus. Such a tank allowstime for the peroxide in the used etchant to decompose before furtherprocessing is performed on the used etch solution to regenerate it.Stabilizing the etch stream obviates concern for subsequent peroxidereactions and related safety issues downstream. The decomposition tankor “etch stabilization” module 610 is appropriately configured to besufficiently large so that the fluid residence time in the tank T islarger than the measured oxidizer breakdown time constant (τ=F/V, whereτ is the average fluids in the tank residence time in minutes, F is thetime average waste etch flow rate in liters per minutes, and V is thebreakdown tanks volume in liters), and generally is not so large as tobe prohibitively expensive or occupy excess volume. The design andvolume of the etch stabilization module should be sufficient so that theexiting solution from the etch stabilization module is less than 50% ofthe inlet oxidizer concentration. In some embodiments, the tank containsvarious means such as flow baffles and cascading barriers to modulatethe flow streamlines, in order to avoid flow short cuts and aids inmaximizing the time in which each new volume waste resides on average inthe tank before exiting the container. Tank 610 contains, for example,cascading barriers. The peroxide breakdown is facilitated by theresidence time (supra) and the physical act of cascading over thebarriers. Tank 610 is vented due to the oxygen liberated during thedecomposition.

As mentioned, hydrogen peroxide or other peroxide is broken downautocatalytically or aided by a catalyst, and in the case of hydrogenperoxide, oxygen gas bubbles are formed, rise, and are separated fromthe fluid. Examples of suitable catalyst include high surface areacarbons such as activated carbon or carbon aerogel, or zeolite supportedhigh surface area platinum or palladium. One additional useful attributeof the etch stabilization cell and the associated exothermicdecomposition reactions is that the exotherm can be used as a heatsource for further processing of the stabilized etch solution. Thetemperature of the solution increases during the decomposition process,as shown in FIG. 1 and indicated by equation (1) and (2). Thistemperature rise can be used to operate the etch process as a whole at ahigher temperature, for example 40-50° C., which can greatly increasethe metal removing etching rate and minimizes the size of the equipmentand heating energy required in otherwise doing so. Put another way, theheat of the decomposition reaction can be used as a chemical energysource to accelerate the etch rate and associated wafer throughput,rather than simply throwing this energy away.

Such decomposition tanks may also have additional features thatgenerally increase the streamline flow path distances. In otherembodiments, the tank contains various means of mixing and recirculatingthe peroxide based etchant prior to exiting the tank so that the tankscomposition is largely uniform. In some embodiments the tank includesone or more suitable oxidizer breakdown catalyst (e.g. activated carbon,noble metal dispersed on a high surface area catalyst), a fluid heightsensor(s), a copper metal ion sensor (e.g. ion specific electrode or aspectrophotometer means of measurements, for example, measuring theadsorption of light at 600 nm) and means of measuring the fluidtemperature in the tank. The metal ion sensor(s) in the system combinedwith the etch flow rate can be used to determine the current metalremoval rate (etch rate) of the wafer. By a mass balance around theconcentration of the incoming and outgoing copper content of the etchsolution and the solution flow rate, the etch rate can be determined,and adjusted as desired to maintain a fixed etch rate, and/or to modifythe etch rate over some predetermine or desired rate/time profile.

Referring again to FIG. 5, after the peroxide is substantiallydecomposed, excess metal is electrowinned from the etchant, see 510.Electrowinning is an efficient mode of handling and treating thestabilized etch waste solution (etch solution where the peroxide hasdecomposed). The etch solution copper concentration of typically around1 gm/L (0.1%, 1000 ppm) is typically too high for cost effective ionexchange treatment alone. Treating the solution in this manner wouldrequire substantial dilution, very large capacity and/or numbers of ionexchange columns and/or frequent column regeneration. The ion exchangetreatment is best suited for remove of metals at concentrations belowabout 200 ppm, or below about 100 ppm. Also, because the etch solutionalready contains complexing agents active in the pH range ofapproximately 7 to 11, ion exchange could require a costly pH adjustmentto more acidic conditions first. If one has a fixed volume waste stream,it makes sense to first remove the bulk of the metal toion-exchanged-suitable levels, so that the total amount of metal to beremoved and the sizing of the ion exchange columns can be minimized.Electrowinning is particularly suited for moderate metals concentrationbecause of its effectiveness and large metal-processing capacity(typically limited, for example, by the porosity of the metal cathodesubstrate). However, because the copper in the solution is highlycomplexed in the alkaline etch waste, it was unclear whether removal ofthe metal by electrowinning was feasible. Surprisingly the inventorshave found that electrowinning the stabilized used etchant is not onlyfeasible, but the copper is efficiently removed and the resultingetchant can be regenerated and reused, in one embodiment, in arecirculation format, for example as depicted in FIG. 6.

In order to test the feasibility of electrowinning the stabilizedetchant, an electrowinning test on a 950 ppm stabilized copper etchwaste solution in a bench scale experimental apparatus was performed.FIG. 4 shows the general layout of an electrowinning apparatus, 400, andFIG. 7 shows the results of the copper concentration in the etch vs.time using the experimental apparatus. The concentration vs. time wasdetermined by taking solution samples periodically and using apre-calibrated spectrophotometric-600 nm-adsorption of the stabilizedetch solution vs. copper concentration. Referring to FIG. 4, the testelectrowinning apparatus included a tank, 405, in this example an 8liter tank (which held about 5 liters of etchant in a flow throughfashion), which had suspended in it alternating pairs of electrodes. A 3Amp/5 volt power supply was used to charge the electrodes. The anodeswere titanium screen coated with niobium. The cathodes were porouscopper “foam” flow through electrodes. A pump, denoted with a “P,”circulated the etchant through apparatus 400 until a desired copperconcentration was reached.

Referring to FIG. 7, the results of the electrowinning test, whileun-optimized (e.g., the number of electrodes used, current densityapplied, total current, flow rate, etc. weren't optimized), clearly showthat removal of metal from the etch solution at the typical copperconcentrations is rapid and can be performed efficiently (small volume,low power, high rate). Moreover, it has been discovered that theelectrowinned etch solution contained essentially all the necessarycomponents of the isotropic etch used to remove the copper, less thehydrogen peroxide oxidizer. It was also realized that, rather thansimply disposing of this processed waste stream, it was highly suitedfor reuse.

The electrowinning process need not be 100% complete (i.e. the copperconcentration of the solution exiting the electrowinning cell forstorage and recycling need not be zero). Rather, at steady state, onlythe balance of the amount of metal added by the etching process must beremoved. A low, but non-zero, copper concentration at the etch moduleinlet will increase by the amount of metal removed in the etch reactor,and that amount of metal on average removed in the electrowinningprocess. This is advantageous because removing metal from a more dilutesolution (e.g., under about 100-200 ppm) is typically slower and hencewould increase the size of the electrowinning equipment for the samefluid flow load. This tendency can be seen, for example, in the data ofFIG. 7, where the rate of copper remove as indicated by the slope of thecurve decreases below about 300 ppm.

Referring again to FIG. 6, the stabilized etchant flow enterselectrowinning module, 615. Although apparatus 600 was described firstin relation to the etching module 605, this was merely a convenientstarting point. In practice, when operating apparatus 600, initiallyelectrowinning cell 615 is charged with concentrated electrolytic etchprecursor solution and water to create the desired concentrations ofisotropic etch constituents (but not the etch oxidizer, typicallyhydrogen peroxide). Examples of suitable isotropic chemical formulationswith concentration ranges and suitable isotropic etching apparatus canbe found in the U.S. patents and patent applications incorporated byreference herein. The etching solution is optionally brought to adesired operating temperature by circulating the fluid in contact with aheat exchanger, collectively heat controller, 635. Heat controller 635may also have a chiller function, as heat is built up in some of theprocesses during circulation of the etchant, for example breakdown ofthe peroxide produces heat which can be used to maintain a steady staterecirculation of optimum conditions in the electrowinning module. Insome embodiments, the electrowinning cell and one or more of the othercomponents described hereafter are located on the tool and in closeproximity to the isotropic wafer etch module. In other embodiments,these etch fluid treating cells are located remotely in the sub-fab andare connected to the tool though appropriate plumbing.

As mentioned, electrowinning cell 615 contains a plurality of porousflow-through cathodes and anodes, as known in the art. The cathode canbe composed of foam metals (e.g. nickel, copper) or carbon (e.g.vitreous reticulated carbon), or other suitable conductive porousmaterials such as conductive screens or clothes that will allow copperto be electrodeposited thereupon. The anode can be made of metals suchas lead, or, for example, a dimensionally stabilized anode (DSA) such astitanium, or coated titanium (e.g. niobium or platinum coated titanium).The stabilized etchant passes over and through the series of porousplates where the copper is plated and removed due to the application ofa potential at the cathode below that of the reduction potential for themetal. Oxygen is the typical (but not necessarily only) generatedproduct at the anode, in the form of bubbles, and some amount ofhydroxide ion is typically formed in the process. Over time this mayaffect the pH of the solution, and some amount of pH adjusting modifiermay be required to be added periodically as well as water and otherconstituents as denoted by one or more feed valves, 645. Bleed valve 640allows portions of the circulating etchant to be removed, for example,after acid and/or salt builds up in the etchant flow and new reagentsneed to be added via the one or more feed valves, 645. The fluid may berecirculated, for example via one or more pumps P1, to allow for it topass over the electrowinning electrodes a number of times and at highvelocities (to improve convection). In preferred embodiments, theelectrowinning cell has various in situ electrolyte monitoringequipment, collectively analyzer 630, (e.g. conductivity, opticaladsorption, pH, density) in the recirculation line capable of measuringthe concentrations of the solution constituents (water, copper, chelatorconcentration, neutralizing agents), and the measured values of thesemeters are used to determine the solution concentrations, which is inturn are used as part of a computer controlled feedback loop to addmaterials to the solution to maintain it at its target values. Theanalysis at the electrowinning module is important, but the addition ofmake up reagents need not be performed at the electrowinning module, forexample, feed stocks can be added at, for example, a buffering tank,620. Also, some of the electrowinned material is continuously orperiodically bled from the system to remove and maintain theconcentration of undesirable breakdown products that may be formed inthe wafer etching, etch stabilization, or electrowinning process (orother sources of stray contaminants) from reaching a level that willimpede the consistent performance and operation of etching process. Thestabilized etchant is circulated through the electrowinning module untilthe desired level of copper is reached. Appropriate valves allow thiscirculation and/or diversion to the next module of apparatus 600.

Referring again to FIG. 6, the stabilized and electrowinned etchant,“etch precursor,” is next passed to an optional etch precursor solutionbuffering tank, 620. This tank allows for a sufficient supply of lowcopper stabilized etch solution to be available at all times, forexample, for a process which may demand material intermittently but inrelatively large volumes, and helps control swings in the volumes offluid stored in the system sub-cells and overall flow loop. One or morerecirculation pumps, P2, are also included that can deliver the etchprecursor to the oxidizer in-line mixing bowl and to the etching cellitself (e.g. for spraying the wafer with etchant). A three way valve,655, is used for switching between recirculation mode and chemicaldelivery mode. In other cases (not shown) two individual values may beused, or only a single value is used which taps fluid off of therecirculating line to deliver etchant to the etch module. Suitabletemperature control means may be integrated into the buffer tank and/orthe recirculation loop, indicated has heat controller 650. Typically thebuffer tank will have a capacity larger than the volumes of the combinedetch cell, stabilizing cell, and electrowinning cell, and is largeenough to hold all the fluid in the entire system. Like thestabilization and electrowinning cells, buffer tank 620 and associatedapparatus may be integrated as part of the wafer processing tool, orlocated more remotely in the wafer processing fab.

Referring again to FIG. 5, process flow 500, the etch precursor is nextregenerated, see 515. Etch precursor is regenerated by combining with,for example, a stream of, for example, about 30% hydrogen peroxide,using an in-line mixing fixture (not shown) and the etch becomes“activated” (capable of etching metal at the target specifications).Adding hydrogen peroxide oxidizer to the stabilized and electrowinningstream regenerates the peroxide-based etchant, and this process ofdecomposing the peroxide from the used etchant, electrowinning thestabilized etchant and regenerating the etchant by adding peroxide canbe maintained, theoretically, indefinitely. In practice, some monitoringof the electrowinning process and solution recycle may be required, aswell as intermittently disposing of some portion of the recycle streamand adding fresh constituents (i.e. bleed and feeding the system).

The amount of time between the point where the fluid is activated and issubsequently exposed to the wafer for etching should be carefullycontrolled, as the isotropic etch solution is inherently unstable, willbegin to heat and release oxygen gas (excess amounts of eitherpotentially leading to an unwieldy process). Typically the fluidresidence time after mixing with oxidizer is kept small by usingrelatively small line length and diameter fluid lines and inline mixingfixture volumes. The residence time between mixing and use shouldgenerally be less than about 3 minutes, more preferably less than about1 minute and most preferably less than about 15 seconds. Valve 625indicates the entry point of the peroxide stream as in close proximityto the etch module 605, in order to reduce residence time.

The regenerated etchant is then exposed to (e.g. sprayed onto) the waferand it etches the metal from the wafer surface, see 520. This completesprocess flow 500. Details of suitable processes and hardware for thisoperation have been described above and disclosed in more detail inpatents and applications incorporated by reference herein. Multiplenozzles can be used to control the instantaneous etching profile, andwhen one or more nozzles are not active, an equivalent flow-resistanceby-pass-line may be used to route the fluid directly to the etchstabilization cell.

Generally, at the concentrations typically employed for the isotropicetch disclosed in the references and examples cited above, and becausethe of the relatively small amount of copper removed compared to theconcentration of the other constituents in the system (0.1% copper vs.typically 1-4% for the active components) the isotropic etch process isrelatively insensitive to the small amount of copper in the regenerated(reconstituted and recycled) etch precursor stream. This attribute caneliminate, in some situations, the need for substantially constant oraccurate copper removal rate and precursor copper concentration as theresults are generally insensitive to the typical process fluctuations.The major impact on the process is on the decomposition rate of theactivated solution and its temperature, and emphasizes the need tominimize the time between the time of fluid activation and utilizationand control the etch temperature. Therefore, in some embodiments,suitable mechanisms for in-line control of the temperature of theactivated etch are envisioned. After etching the wafer the etch fluid iscollected at the etch module drain and directed to the etchstabilization module. The wafer is spun and rinsed at the end of theprocess. Various modes of minimizing the amount of rinse water that endsup, and potentially would dilute the etch recovery system are, forexample, a rinse diverter valve, and/or a multiple high cellconfiguration with separate level or other diverters preventing therinsate from diluting the etchant stream.

While apparatus 600 is shown as a continuously flowing system, one ofordinary skill in the art would appreciate that apparatus 600 is notlimited in this way. For example, the stabilized waste from the peroxidedecomposition tank can be stored in a buffering tank prior to beingelectrowinned, and the electrowinning need not be continuously operated.Similarly, the product of the electrowinning copper removal can bestored in a “buffering tank” as show in FIG. 6. The recovery system andrecycle loop can have one or more modules integrated into a work pieceprocessing tool such as a semiconductor manufacturing tool thatprocesses wafers, or the recovery system can have one or more componentsoff line of such a tool. For example, the electrowinning system can beperformed on or off the tool. In some embodiments, the stabilizationsystem is on the tool, and the electrowinning system is fed fluid fromthe stabilization system, but is located in a remote location within thesame facility (along with an optional remotely located etch precursorbuffer storage tank). In other embodiments, both the electrowinningsystem and the stabilization system are located and perform theirfunctions on the tool and relatively close proximity to the etchingmodules. In still further embodiments, the etch solution is stabilizedand then transported off site, where it has its metal removed byelectrowinning, ion exchange, or other suitable process, and thenreturned to the facility to a recovered etch precursor storage tank thatfeeds the in-line mixing system.

The system which bleeds part of material in the flow loop as part of ableed and feed operation generates an etch waste stream (not necessarilyhaving to come from the location in the flow loop shown in FIG. 6). Sucha waste stream can be combined with an existing rinse water wastestream, treated plating wastes, or any other waste stream, and routed toundergo subsequent treatment processes. Typically, if one combines aplating rinse stream and an electrowinned plating stream with theelectrowinned etch streams, these streams would undergo furthertreatments, such as carbon filtration and ion exchange, before beingdischarged to municipal waste.

Numerous other modifications may be made to the foregoing system withoutdeparting from the basic teachings thereof. Although the presentinvention has been described in substantial detail with reference to oneor more specific embodiments, those of ordinary skill in the art willrecognize that certain changes may be made, all without departing fromthe scope of spirit of the invention as set forth in the specificationsand in the appended claims.

1. An apparatus for processing a work piece, comprising: (a) a wetchemical etching chamber for removing a metal from a surface of the workpiece with a peroxide-based etchant, said wet chemical etching chamberin fluid communication with and upstream of; (b) an electrowinningmodule for removing ions of the metal from the peroxide-based etchantafter it exits the wet chemical etching chamber; and (c) a regenerationsystem configured to reintroduce one or more reagents into theperoxide-based etchant after it exits the wet chemical etching chamberin order to regenerate the peroxide-based etchant and reintroduce itinto the wet chemical etching chamber.
 2. The apparatus of claim 1,further comprising a decomposition tank configured to allow substantialdecomposition of a peroxide in the peroxide-based etchant after exitingthe wet chemical etching chamber and before entering the electrowinningmodule.
 3. The apparatus of claim 2, wherein the regeneration systemcomprises: a peroxide inlet for introducing the peroxide into theperoxide-based etchant after the peroxide-based etchant exits theelectrowinning module, said peroxide inlet proximate to an etchant inletof the wet chemical etching chamber; and one or more make-up feeds foradding at least one of water, a metal chelator and a pH adjuster to theperoxide-based etchant.
 4. The apparatus of claim 3, further comprisinga buffer tank configured to store a volume of the peroxide-based etchantafter it exits the electrowinning module, said buffer tank configuredupstream of the peroxide inlet.
 5. The apparatus of claim 3, wherein thedecomposition tank is configured to hold the peroxide-based etchant forsufficient time such that greater than about 50% of the peroxide in theperoxide-based etchant decomposes without applying heat or additionalreagents to the peroxide-based etchant in the decomposition tank.
 6. Theapparatus of claim 3, wherein peroxide-based etchant comprises an aminemetal chelator and the peroxide is hydrogen peroxide.
 7. The apparatusof claim 3, wherein the metal is copper and the work piece is asemiconductor wafer.
 8. The apparatus of claim 7, wherein theelectrowinning module is configured to remove copper ions from theperoxide-based etchant so that the peroxide-based etchant exiting theelectrowinning module comprises less than about 200 ppm of copper ions.9. The apparatus of claim 7, wherein the electrowinning module isconfigured to remove copper ions from the peroxide-based etchant so thatthe peroxide-based etchant exiting the electrowinning module comprisesless than about 100 ppm of copper ions.
 10. The apparatus of claim 3,configured to deliver regenerated peroxide-based etchant to the wetchemical etching chamber in less than about 3 minutes.
 11. The apparatusof claim 3, further comprising a bleed valve for removing theperoxide-based etchant from the apparatus to compensate for said one ormore make-up feeds adding said at least one of water, a metal chelatorand a pH adjuster to the peroxide-based etchant.
 12. The apparatus ofclaim 3, further comprising a buffer tank configured to store a volumeof the peroxide-based etchant after it exits the decomposition tank,said buffer tank configured upstream of the electrowinning module. 13.The apparatus of claim 11, further comprising one or more analyticalprobes configured to measure the peroxide-based etchant in theelectrowinning module for at least one of pH, salt concentration,peroxide concentration, metal ion concentration, ionic strength andchelator concentration.
 14. The apparatus of claim 13, furthercomprising a controller comprising an associated logic, said controllerconfigured to operate at least the bleed valve and said one or moremake-up feeds in order to maintain a steady state in the composition ofthe peroxide-based etchant as it exits the electrowinning module usingmeasurements obtained from the one or more analytical probes.
 15. Amethod for processing a work piece, comprising: (a) wet chemical etchinga metal from a surface of the work piece with a peroxide-based etchant;(b) electrowinning ions of the metal from the peroxide-based etchantafter wet chemical etching; (c) regenerating the peroxide-based etchantby adding one or more reagents to the electrowinned peroxide-basedetchant; and (d) reusing the regenerated peroxide-based etchant for wetchemical etching.
 16. The method of claim 15, further comprising passingthe peroxide-based etchant through a decomposition tank, after wetchemical etching and before electrowinning, in order to decompose aperoxide in the peroxide-based etchant.
 17. The method of claim 16,wherein greater than about 50% of the peroxide in the peroxide-basedetchant decomposes without applying heat or additional reagents to theperoxide-based etchant in the decomposition tank.
 18. The method ofclaim 16, wherein greater than about 50% of the peroxide in theperoxide-based etchant decomposes and a decomposition catalyst is addedto the peroxide-based etchant.
 19. The method of claim 15, wherein themetal is copper.
 20. The method of claim 19, wherein electrowinning isperformed until the peroxide-based etchant comprises less than about 200ppm of copper ions.
 21. The method of claim 19, wherein electrowinningis performed until the peroxide-based etchant comprises less than about100 ppm of copper ions.
 22. The method of claim 15, wherein said one ormore reagents comprises at least one of water, a peroxide, a metalchelator and a pH adjuster.
 23. The method of claim 22, wherein theperoxide is hydrogen peroxide.
 24. The method of claim 22, wherein themetal chelator is an amine.
 25. The method of claim 15, furthercomprising removing a portion of the peroxide-based etchant tocompensate for the addition of said one or more reagents to theelectrowinned peroxide-based etchant.
 26. The method of claim 25,further comprising: determining which, and how much of each, of the oneor more reagents to add to the electrowinned peroxide based etchant byanalyzing the electrowinned peroxide-based etchant to determine at leastone of pH, salt concentration, peroxide concentration, metal ionconcentration, ionic strength and chelator concentration; and addingsaid one or more reagents to the electrowinned peroxide based etchantand/or removing said portion of the peroxide-based etchant in order tomaintain a steady state composition of the electrowinned peroxide basedetchant.
 27. The method of claim 15, wherein (a)-(d) are performed in arecirculating fashion.